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Lecture 6

BIOB11H3 Lecture 6: 2017 OP6&7(1)


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dan Riggs
Lecture
6

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OP6/7: Elucidation of the genetic code/protein synthesis
Smart Pages: Figs 2-24; 11-37, 38, 39, 40, 42, 44, 45, 47, 50
Vocabulary Words: translation/codon/frameshift/tRNA/Anticodon/wobble base/amino acyl
tRNA synthetase/Initiation codon/Shine-Dalgarno sequence/elongation/A,P,E sites/peptidyl
transferase/translocation/polysome
Central ideas:
1. in eukaryotes, mRNAs are exported to the cytoplasm where they are decoded into
protein by ribosomes.
2. the ‘genetic code’ is a triplet code, with each triplet set of nucleotides in RNA
decoded into an amino acid. Three of the 64 codons are ‘stop’ codons that signal
the end of the polypeptide chain.
3. Transfer RNAs (tRNAs) serve as adaptors for this process. The anticodon of a tRNA
forms complementary base pairs with the triplet codon in mRNA.
4. Aminoacyl tRNA synthetases recognize the structure of each tRNA and add the
appropriate amino acid to the 3’ end of the tRNA.
5. After initiation, which is regulated by specific proteins, cycles of elongation add
amino acids to the growing chain.
6. The ribosome possesses A, P, and E sites where distinct activities occur.
7. Translation is an energy requiring process where GTP is used at several steps.
Questions a scientist might ask:
1. Since most mRNAs have a 5 untranslated region, how is the start site known?
2. What is meant by redundancy of the code? How does the wobble theory relate to
this?
3. What happens if an incorrect tRNA/amino acid comes to the ribosome?
Sample multiple choice question:
The prokaryotic translation factor Tu:
a. Acts to specify chain initiation by binding to the start codon.
b. Catalyzes peptide bond formation.
c. Brings an aminoacyl tRNA to the ribosome.
d. Recognizes the tRNA and adds the appropriate amino acid
e. None of the above.
Analytical Questions:
1. What could be the consequences of incorporation of the wrong AA into a protein?
2. What is the experimental evidence that multiple ribosomes act simultaneously on
the same mRNA?
3. See questions 1-3, page 439; questions 1-2, page 442, questions 1-2, page 445,
questions 1-2, page 448, questions and questions 5, 8-12 on page 454.
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